1. Field of the Invention
This invention relates generally to apparatus for reproducing optically an information signal from a record disc rotated thereon, and more particularly to an optical disc player operative to cause a light beam to impinge through an object lens on a recording surface of a record disc and to receive a reflected light beam coming through the object lens from the record disc by a photodetector so as to read an information signal recorded on the record disc.
2. Description of the Prior Art
In an optical disc player for reproducing with a light beam an information signal from an optical record disc having a recording surface on which a spiral record track composed of an arrangement of variations in geometrical shape, such as a plurality of pits, formed therein is provided, the optical record disc is rotated and it is necessary to provide a focus servocontrol arrangement for performing focus servocontrol to maintain correct focus of the light beam projected onto the recording surface of the rotating optical record disc and a tracking servocontrol arrangement for performing tracking servocontrol to maintain the light beam in correct tracking relation to each turn of the spiral recorded track on the rotating optical record disc. Besides, in the case that a reproduced information signal obtained from the optical record disc is required to have a strictly correct time base, that is, the reproduced information signal is a video signal from an optical video disc, for example, it is also necessary to provide a time base correcting servocontrol arrangement for performing time base correcting servocontrol to adjust the light beam to scan such locations on the spiral record track that the reproduced information signal is obtained with a predetermined correct time base, in addition to the focus and tracking servocontrol arrangements.
With the focus servocontrol arrangement, for example, a position of the object lens, which causes the light beam to impinge on the recording surface of the optical record disc, in the direction perpendicular to the recording surface of the optical record disc is controlled to maintain a distance from the object lens to the recording surface in such a state that the light beam is correctly focused on the recording surface. With the tracking servocontrol arrangement, for example, a position of the object lens in a plane parallel with the recording surface of the optical disc is controlled to maintain a location where the light beam impinges on the recording surface on each turn of the spiral record track in the recording surface correctly. Further, with the time base correcting servocontrol arrangement, for example, the position of the object lens in the plane parallel with the recording surface of the optical record disc is controlled to be moved in a direction perpendicular to the direction of the shift caused by the tracking servocontrol arrangement to shift the location where the light beam impinges on the recording surface in a direction along each turn of the spiral record track in the recording surface.
In such a manner as mentioned above, several servocontrols to the light beam impinging through the object lens on the recording surface of the optical record disc are carried out, so that a reproduced information signal is properly obtained. However, notwithstanding such servocontrols, when an optical axis of the light beam impinging through the object lens on the recording surface of the optical record disc is inclined with respect to a normal line perpendicular to the recording surface at a location where the light beam impinge on the recording surface due to deformations of the optical record disc, for example, (such an inclination of the optical axis of the light beam with respect to a normal line perpendicular to the recording surface of the optical record disc at a location where the light beam impinges on the recording surface will be referred to as a skew, hereinafter.) and therefore there is a certain angular space between the optical axis of the light beam and the normal line at the location where the light beam impinges on the recording surface, a spot of the light beam on a certain turn of the spiral record track in the recording surface is undesirably enlarged by coma abberations caused in connection with the object lens, and as a result of this, a reproduced information signal contains increased crosstalk components from adjacent turns of the spiral record track and the frequency characteristic of the reproduced information signal is deteriorated. In particular, in the case where a semiconductor laser device is employed to act as a light source for the light beam, there is a disadvantage that remarkable deteriorations in the reproduced information signal result from even a relatively small skew because a wavelength of a laser light emitted by the semiconductor laser device is relatively long (for example, considerably longer than a oscillation wavelength : 6320 angstroms in the case of a Helium-Neon gas laser device) and an object lens having a relatively large numerical aperture is used for causing such a laser light to focus correctly on the recording surface of the optical record disc, so that the permissible amount of skew is reduced.
In view of the above, it is desired to carry out such a control as to cancel the skew by, for example, varying the direction of the optical axis of the light beam impinging on the recording surface of the optical record disc when the skew occurs. For performing such a control to cancel the skew in the optical disc player, the optical disc player is required to have means for detecting with certainty the skew caused therein and means for producing a signal responding accurately to the amount of detected skew. However, any optical disc player having satisfactory means for detecting the skew and for producing the signal responding to the skew anount has not been previously known.